The Old Gods are immortal more in the sense that they aren't and never were mortal, physical destruction of an Old God seems to a pretty ineffective method of "killing" them. It seems that the only way to destroy an Old God completely is to obliterate their essence - something which may or may not be possible (the Old Gods, to me, seem to be the very avatars of the elemental force of Corruption). As long as Corruption exists as a concept so might the Old Gods, even if their forms are destroyed and their essence dispersed. I think that the perverse plans of the Old Gods actually *hinge* on our wholesale destruction of their physical forms and intermediaries, I think the Titans made the same mistake we do in thinking of them as purely physical lifeforms - they imprisoned them physically in various places, but in slaying them we effectively release them from their prisons and allow them to get on with the job of infecting nigh everything in existence.

Everyone has a story they tell themselves to justify bad decisions - and it never matters. In the end, you are always alone in your actions.

The Sha are also far from gone. If their related emotions continue to run rampant on pandaria, after enough generations they'll simply start to manifest once more.

The Sha's have been dealt with.. Once Sha of Pride is dead then the Sha's will never be seen manifesting again.

For years on Pandaria the Pandaren started to question whether the Sha even exist, but they kept the stories going because the ironic fear of them is what kept peace and harmony there. If a Sha did manifest, then I would say it would be little more than a minion, one of the aberration's, never THE Sha of what ever particular emotion is being drawn from.

The reason why all the Sha's have suddenly appeared to us is because we brought so much emotion to Pandaria's shores, enough of it to herald the actual Sha's and not their minions. Blizzard have said that the Alliance and Horde are directly responsible for the Sha's manifesting, and now we have killed them.

Originally Posted by tweekzlol

but then again, The Coon is obviously unbeatable as he will just manipulate you into being his friend

I don't believe they can truly be killed, though the lore is purposely vague about this. I like to think of the Old Gods as having an existence beyond their physical forms. Maybe we'll be able to figure out more once Blizz clarifies how they are connected to Azeroth and why killing them is bad for the world.

Originally Posted by Lord Pebbleton

Y'shaarj proves they can be killed.
Y'shaarj also proves they can be resurrected at will.
I'd say they are like bugs; you stomp on one, three more appear.

Originally Posted by Dave Kosak

What are some of the larger implications that will come into play for having a dead Old God on Pandaria, if any, and is there a chance we might get to see some form of its body?
I'm not going to answer if we're going to see his body or not. But sort of the Old God is a lot of this negative energy that has become the Sha that still haunts and pollutes this land. Ramifications... Not every Old God is still alive and plotting. The Titans actually did kill a lot, so this particular Old God is dead. And is safely dead. Not all of the Old Gods were entombed. (Source)

At the end of the Klaxxi questline, the mantid mentioned that at one point in time, they worshipped Y'shaarj, an old god. However, he was slain by the Titans and was split into "shadows of his former self". Is he, in some way, in relations with the Sha and will we see Y'shaarj in future content patches?
Y'shaarj breathed love and exhaled hate, inhaled peace and breathed violence... Plus, his name was y'SHAarj. It's pretty safe to connect the dots. :smileyhappy: But Y'shaarj itself is very, very, very dead. (Source)

Why did Y'shaarj side with Garrosh according to Sound Files? Is Y'shaarj an idiot!?!
Garrosh brings heart of Y'shaarj back to life using the pools of Pandaria. Fortunately, the whole God isn't resurrected. (Source)

Kosak says Y'shaarj is safely dead, but then we see his Echo attacking us even before Garrosh takes the heart; ultimately, Garrosh partially revives him. While it didn't happen fully, it's still a resurrection.
Is there something i've missed?